Field of the Invention
The present invention relates to a light source device such as a broadband light source and to an information acquisition apparatus having the same.
Description of the Related Art
In recent years, as a coherent broadband light source (light source device) for a medical imaging apparatus, broadband light sources that utilize a nonlinear wavelength conversion technology by using a highly nonlinear optical fiber have been actively studied. A medical imaging apparatus in which such a broadband light source is used may be a spectrally encoded endoscopy, for example. In addition, an optical coherent tomography is an example thereof.
When high-intensity light propagates in a highly nonlinear optical fiber, a nonlinear optical effect of the highly nonlinear optical fiber causes the propagating light to be converted into a super continuum light (hereafter, referred to as SC light) that is a broadband light having a wide wavelength band. As a highly nonlinear optical fiber used for such wavelength conversion, a photonic crystal fiber having periodical micro-machined features provided in a core and cladding of the optical fiber may be used. Periodical micro-machined features are provided in a core and cladding of a photonic crystal fiber. With this structure, an intense light can be confined in the core of the photonic crystal fiber, and a high optical density that is necessary for effective wavelength conversion can be obtained in the core.
Further, the dispersion of the photonic crystal fiber can be controlled by changing the shape of micro-machined features of a photonic crystal fiber. In general, in order to efficiently generate an SC light, it is desirable that the wavelength of an incident light to a highly nonlinear optical fiber be a wavelength that is close to the zero dispersion wavelength of the highly nonlinear fiber and has an anomalous dispersion. An adjustment of the dispersion is easier in a photonic crystal fiber with periodical micro-machined features than in a photonic crystal fiber without periodical micro-machined features. Therefore, an SC light can be effectively generated by taking the wavelength of an incident light into consideration and selecting a photonic crystal fiber having appropriate structure.
As an incident light to a photonic crystal fiber, a mode-locked pulse light having a pulse width of 100 ps or less may be used. A use of a pulse light having a pulse width of 100 ps or less makes it easier to obtain a light having a high peak intensity required in generating a nonlinear optical effect. Further, a use of a mode-locked pulse light allows for a broader band of the wavelength of a light propagating with various nonlinear optical effects. A nonlinear optical effect which contributes to broaden a band may be self-phase modulation, soliton fission, soliton self-frequency shift, non-solitonic radiation, and pulse trapping, for example.
Note that mechanisms of an SC light generation caused by a pulse light propagating in a photonic crystal fiber are disclosed by Govind Agrawal, “Nonlinear Fiber Optics”, Fifth Edition, Academic Press (2012), Samudra Poy et al., “Dispersive wave generation in supercontinuum process inside nonlinear microstructured fibre”, Current Science, Vol. 100, No. 3, pp. 321-342 (2011), and Haohua Tu et al., “Optical frequency up-conversion by supercontinuum-free widely-tunable fiber-optic Cherenkov radiation”, Optics Express, Vol. 17, No. 12, pp. 9858-9872 (2009). Further, a resonator-type broadband light source that generates an SC light is disclosed in U. S. Patent Application Publication No. 2013/0259071.
Some light source devices that generate an SC light have dispersion compensation optics. In such a light source device, however, the presence of dispersion compensation optics may cause problems such as an increase in size of the light source, complicated alignment of components, and instability of the light source due to misalignment.